The prior art to which the invention is directed includes the art of valves and particularly butterfly valves being a type in which a usually circular closure vane or disc is mounted either centralized or offset for rotation in the body flow passage between an open and closed position. Construction of such valves commonly consists of a body and/or closure vane of cast iron or the like operable by means of an operator shaft extending through a bore in the body wall to externally thereof. Where the valve is likely to be utilized in corrosive service, the more corrosion vulnerable components and sometimes the entire valve is frequently constructed from more exotic and more expensive materials such as stainless steel. As an alternative less costly construction, a recent trend in the industry has been to apply either a protective polymeric coating and/or a detachable liner about those components or surface portions susceptible to corrosive attack by the line content of the piping system in which the valve is to be used. Exemplifying such prior art type butterfly valves for corrosive service of both the centralized and offset variety are the disclosures of U.S. Pat. Nos. 3,958,595; 3,801,066; 3,563,510; 3,425,439; 3,376,014; 3,241,806; and 3,076,631.
Disclosed in U.S. Pat. No. 3,958,595 is a unique construction in which the entire valve, including the shaft bore, is coated for protection. Not only is the entire valve thereof afforded corrosion protection enabling use of the less costly metals that would otherwise be susceptible to corrosive attack, but the bore coating per se can for appropriate applications be rendered capable of carrying the bearing load of the rotatable vane shaft. While the bearing capability of a nylon bore coating has proven to be highly suitable for bearing loads of up to about 4000-5000 psi, it has been found that at encountered loads generally exceeding those limits the coating is insufficiently durable to afford the required bearing life normally expected of such valves. At the same time, servicing such valves from the standpoint of bearing maintenance can be most difficult. The need for enhanced bearing capacity under those circumstances has therefore been recognized, but unfortunately conventional approaches have tended to destroy continuity of the protective coating to in turn defeat the corrosive protection intended to be afforded thereby. Despite recognition of the problem, a ready solution of how to enhance bearing capacity without destroying the protective coating has not heretofore been known.